1. Field of the Invention
The present invention relates to a speed change control system for an automatic transmission, particularly a transmission control system that sets the gear ratio of the automatic transmission to a gear ratio that matches engine operation conditions, vehicle speed and load of the vehicle
2. Prior Art of the Invention
In the conventional automatic transmission which is electronically controlled, for example, in the transmission disclosed in the Japanese Laid-open Patent Publication No. 35858/1980 (Patent Application No. 11192/1979), the gear ratio is set by the combination of the opening and closing of a plurality of solenoid valves in the hydraulic circuit which are caused to be opened or closed by the electronic control system connected to the torque converter with a direct-coupled clutch. The electronic control system measures the load of the vehicle from the engine operation condition (the opening of the throttle in the case of the embodiment of the invention), vehicle speed and the position of the shift lever, while the vehicle is running. Then the system corrects the stored data for the judgment of the gear ratio according to the load of vehicle, compares the detected value with the corrected data to calculate the intended gear ratio, and sets the automatic transmission to the intended gear ratio to control the speed change. Furthermore, said electronic control system enables the transmission to be locked up at any gear ratio, instead of locking up the transmission at a specific high speed gear ratio such as 3rd gear or overdrive. Thus, the transmission is set in a torque converter condition to prevent lowering of the power performance in case where the speed change performance of the torque converter is effective, and is locked up in case where the speed change performance of the torque converter is not effective. Such lockup control is applied to each speed change step, thereby extending the range of the lockup and enabling a reduction of fuel consumption.
On the other hand, almost all conventional automatic transmissions are designed for starting at a low speed (first gear). However, starting at a high speed (e.g. 2 nd speed) is allowable under some condition of the load of the vehicle (including the change in the load in climbing). Therefore, some automatic transmissions are so designed as to allow starting with a high gear ratio, as it allows for fast acceleration, the elimination of the shock occurring at the time of the shift from the low-speed gear to high-speed gear and the reduction of fuel consumption. But, with this type of automatic transmission, it is necessary for the driver to select the range exclusive for high-speed starting by operating the shift lever. In this case, the transmission is held at the high speed stage by changing the hydraulic pressure through the manual valve of the automatic transmission operated by the shift lever. Therefore, starting at low speed is impossible when the transmission is set in the range for starting with high gear ratio, and the driver is required to operate the shift lever each time when selecting start at a low speed or starting at a high speed.